Physical servers are able to provide service to users. For example, in response to a user request from a terminal device over a network, a physical server operates at the request. Such service provision is realized by using, for example, virtual machines.
Virtual machines (VMs) are virtual computers that are created by virtualizing resources such as Central Processing Units (CPUs) and storage devices of physical servers.
Virtualization of computer resources enables a plurality of virtual machines with different Operating Systems (OSes) and software to run on a single physical server simultaneously, which leads to efficient use of the physical server.
In addition, such a server system employs a technique (live migration) of placing a virtual machine on another physical server without stopping service provided by the virtual machine. Placing the virtual machine causes the physical server to run the virtual machine. Appropriate virtual machine placement enables power saving and load balancing in the system.
For example, power saving is realized by placing virtual machines on specified physical servers according to variations in the loads on the CPUs and memories of the physical servers and suspending physical servers where no virtual machines are located (Distributed Power Management (DPM)). On the other hand, load balancing is realized by migrating virtual machines from a physical server with a high load to a physical server with a low load (Distributed Resource Scheduling (DRS)).
In relation to virtual machine placement, there has been proposed a technique of selecting a physical server that satisfies constraint conditions for placing virtual machines and has the highest priority. There has also been proposed a technique of calculating the reliability characteristics of a virtual system in which virtual devices are located on physical devices, on the basis of system configuration information indicating a correspondence between the virtual devices and the physical devices.
Please see, for example, Japanese Laid-open Patent Publications Nos. 2011-13822 and 2008-293103.
In determining the location of a virtual machine, i.e., determining which one of chassis housing physical servers and which physical server of the chassis the virtual machine is to be placed on, a conventional technique is to select a physical server with a relatively low load with the consideration of the loads on the CPUs and memories of the physical servers and place the virtual machine thereon.
However, if an error such as power interruption, disconnection of server communications, etc. occurs in a chassis where a virtual machine is located, there arises a problem in which the virtual machine stops its execution and therefore the server system stops, which reduces availability (sustainability of operations). Moreover, if such an error occurs in the chassis, not only service provided using the virtual machine but also service provided by physical servers housed in the chassis stops, which reduces the availability.